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TRANSCRIPT
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BY
ANDREA LANARI*
SYNOPSIS:
After 7 years and more than 10 installations worldwide, the results achieved by
Condoor® –automatic slag door by SMS Group- are presented as well as the lessons
learnt that led to the present technical solution through three major steps, summing up
hundreds of technical improvements, each born from steelmakers’ experiences and
requirements.
Starting point for the new developments are the proven performances achieved and
the fast Return-On-Investment, driven by remarkable savings in:
- Energy consumption
- Electrodes consumption
- Power-off time
Developed essentially for safety requirements, Condoor® turned out to be an efficient
solution and a key Technological Module of the modern Electric Arc Furnace concept
by SMS Group. In the past years a survey of the plants was carried out in order to
define the new roadmap for its constant upgrade. Today, SMS Group counts more
than ever on Condoor® to provide a safer, sustainable and efficient electric steel
making process.
This paper describes the current status of the installations and introduces new
technical details and operating results. A specific focus is dedicated to Electric Arc
Furnaces based on flat bath operations, where the proper control of de-slagging is a
key factor for process and operation quality.
Keywords: Electric Arc Furnace, Safety, Energy Saving, Efficiency, Operational
Results, Residence Time, Slag Door, Deslagging, Power Off,
Consistency
*Area Sales Manager, Metallurgical Plants and Environmental Technologies - SMS
group S.p.A., Tarcento (UD), Italy
AUTOMATIC SLAG DOOR PRACTICE: ACHIEVEMENTS AND
DEVELOPMENTS BASED ON 7 YEARS OF EXPERIENCE
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Introduction: conventional slag door – a hole in the EAF?
Slag door is a part of EAF that has not been changed since many decades. Except
for a few minor changes in dimensions and construction, slag door remains mostly
unaltered. While alternatives for some of its functions have been devised, its main
function –deslagging- remains quite unchallenged. Use of alternative iron sources like
Hot Metal (HM) and DRI/HBI has remarkably changed the needs of the process over
the years. Looking at the way EAF slag volume has gone up in the last few decades
and the process time has been drastically reduced, it is surprising that the method of
deslagging has not gone through any major upgrade. One of the basic limitations of
the slag door lies in its poor control of deslagging which still depends on the Operator’s
skill and judgement. With little control over its shape, it is nearly impossible to know
how much slag has been lost or retained at any point of time. There is always a
possibility of metal seeping through the “V” channels formed on the breast. The
quantity of metal found in the slag pots often tells that this process of deslagging is not
very accurate. Furthermore many automatic machines and equipment are found
nowadays on the working platform close to the EAF, insisting on the slag channel area.
Sampler, Oxygen lances, Carbon injection, Hot
metal launder require the slag channel to keep
a proper shape: incorrect sill profile may result
in incorrect or unsuccessful operations, thus in
an increased power off time.
Above all, there can be no argument that slag
door and EBT areas remain the most unsafe
zones in EAF Area.
Loss of Fluxes and Yield
Compared to scrap melting furnaces, EAFs with longer flat bath operation often
generate more slag. The EAFs using HM/DRI/HBI must provide enough flux to
neutralize the acidic content coming from the burden. Typically DRI with higher gangue
content requires bigger quantities of basic fluxes. With different plants using different
qualities of DRI/HBI, it is difficult to compare the process parameters like flux
consumption and Liquid Metal (LM) yield. Within the MENA Region itself, the plants
using 100% DRI/HBI have been remarked using lime and dololime quantity varying
from 25 kgs to 120 kgs per Ton of steel. Of course, the main reason for this is the
different quality of input DRI/HBI in terms of gangue content and quality.
This is true not only in the same region, but in the same plant too, as easily understood
from the graph below taken from a 10 months analysis on a DRI process based 120
Tons AC furnace.
Fig.1: Slag channel obstruction
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Fig.2: Slagbuilders consumption on a 120 Tons EAF DRI based
We tried to analyze whether the DRI/HBI quality is the only variable affecting the
consumption of lime and dololime.
Process data of 425 consecutive heats from a meltshop in MENA region was analyzed
to examine the utilization of fluxes towards slag making. The meltshop - designed and
supplied by SMS Group - has an EAF of 120 Tons capacity using a combination of hot
and cold DRI received from a captive DRI plant. The basicity of the oxides portion of
DRI was more than 0.20 during this period. The theoretical basicity of slag, IB3T was
calculated from the mass balance of EAF taking into account incoming and outgoing
quantities of CaO, MgO and SiO2. The losses of slag builders through exhaust gases
were considered. The actual slag basicity, IB3A, is recorded by slag samples taken
during the heats.
IB3T = {(CaO +MgO)/SiO2} calculated from material data.
IB3A = Basicity of slag sample taken during the heat
The graph plotting values of IB3T and IB3A show that a significant part of flux addition
is not reflected in the slag sample, see Figure 3. From the mass balance of the heats,
the percentage of missing lime was 25.7! This means one quarter of the added lime
fails to dissolve and mix with the slag. This loss of lime can be explained by a variable
called “Residence Time” of slagbuilders. Considering an infinitely long Residence
Time, the two basicities would be equal. But a smaller Residence Time would allow
only a certain degree of mixing and reaction in the slag resulting in a gap.
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Fig.3: Theoretical and Actual Slag Basicity in EAF
What are the factors affecting the Residence Time of slag builders?
If two geometrically identical EAFs, namely A and B, have equal process time, and the
lime consumption of A is higher than that of B,
Residence Time of lime in B > Residence Time of lime in A
If two EAFs have equal process time and equal lime consumption and their slag
holding capacities are different, the EAF with bigger volume for slag will have a longer
Residence Time. In other words, if the slag holding capacity of an EAF is increased,
the Residence Time of lime increases at the given process time.
Therefore, the Residence Time of any slag builder, K, in electric arc furnace can be
defined as the ratio of the total volume utilized by slag during process and the
volumetric input flow rate or the consumption rate of K. This is true for all slag builders
such as lime, dololime and the oxides in DRI. Compared to conventional scrap melting
process, EAF process based on Hot Metal and DRI involves much lower Residence
Time for slag builders because of higher slag generation with similar slag holding
capacity of reactor.
Another factor that plays a significant role is the distribution of Residence Time of the
slag builder. In a plug flow reaction vessel, the Residence Time for all the particles of
slag builder is equal. But in EAF, at the time when material addition and deslagging
happen simultaneously, the particles of slag builder which follow the shortest route
between the entry and exit points have considerably less Residence Time than others.
This is very important in smaller EAFs where the distance between lime feeding point
and slag door is often less than the radius of the vessel. This is equally important for
the EAFs that use slag door for DRI Injection.
For EAFs using Hot Metal Charge the challenge of retaining slag is equally big. Loss
of lime-rich slag retained from the previous heat is a common event when HM is
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1.5
2
2.5
3
3.5
4
0 50 100 150 200 250 300 350 400 450
Theoretical Basicity
Actual Basicity
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poured into the low carbon hot heel and oxidizing slag. For continuous scrap charging
in EAF, volume of slag cannot be increased beyond a certain limit, due to design
constraints, and lost volume of slag must be restored by continuously adding slag
builders.
EAF has a distinct advantage of ultra-
high temperature in the feed zone of
lime and DRI due to electrical arcs.
This temperature is much lower in
case of BOF and EOF making the
process of dissolution of lime much
slower. Due to the presence of an
EAF-like slag door, the residence time
for slag builders in EOF is considerably
less than that in BOFs. And that is
perhaps the reason behind the lower
LM yield in an EOF when compared to
a BOF.
Fig.4: Slag Volume and Residence Time
Poor Atmosphere Control
Conventional slag door has always been the biggest source of cold air ingress in EAF.
Since most of the EAFs are not recovering heat from the waste gases, this deficiency
is often ignored. But this can be very crucial in case of scrap preheating by use of
exhaust gases. Many of such EAFs can improve their performances if the air ingress
can be controlled.
Therefore, the challenges for a new approach to slag door operation are:
1. control over the slag volume
2. control over the atmosphere
The old fashioned approaches to slag door management do not meet the above
requirement. A forklift with ram can initiate the process of deslagging in a few seconds
but it cannot stop the flow of slag upon request of the Operator. Lifting the sill level by
depositing basic material or by adding artificial barrier on the door is both unsafe and
time consuming.
One of the solutions for enhancing the Residence Time of slag builders in EAF is
Condoor®.
What is Condoor®?
Condoor® (former ESD, Enhanced Slag Door) is a two axis robot developed by the
SMS Group from a joint idea with the steelmakers. It can act as the perfect valve for
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slag control and it can push any jam on sill inside the shell with its hydraulic force of
24 Tons at 120 Bar. Its water cooled pusher not only cleans the tunnel, but actually
can sit on the door tunnel sealing it completely. It can withstand mechanical erosion
of slag and radiations from arc. Thanks to Condoor®, the Operator can choose the
exact time of deslagging as the door can be opened in a few seconds. Forklift and/or
Operators do not need to be present in front of the slag door any longer.
Fig.5: Evolution of Condoor® (a) Version 1.0 (b) Version 1.5 and (c) Version 2.0
The first Condoor® installed in Nucor Jewett in late 2009 (aka Texas Tailgate) raised
many eyebrows due to its big and sturdy design. Soon it was discovered that a pushing
stroke more than 300 mm was not enough: furthermore the force itself was a factor
underestimated at that time (only 6 Tons at 120 bar). In the very next version, the need
for a stronger pushing force with bigger stroke was targeted. Since then, through its
several upgrades, Condoor® has come a long way: today’s design enables a pushing
stroke of 800 mm with 24 Tons force and a pulling force of 16 Tons. It has been tested
in different kinds of steelmaking process ranging from structural steels to stainless
steels. It has been tried successfully for different kinds of iron sources ranging from
scrap to Hot Metal.
One feature that sets Condoor® apart from any other slag door solution is the design
of the pusher. It has become evident that the control over the slag door is not possible
without the capability to control the door opening from 0 to 100% and back to 0%.
Fig.6: Condoor® (a) 100% closed (b) fully open showing perfect rectangle
The pusher is the most exposed part of the equipment, subject to wearing due to heat,
radiation and slag actions. It goes without saying that each furnace has its own story
but some common lessons learnt have driven to the present solution based on a full
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copper water-cooled front panel. For less aggressive environment, replaceable
stainless steel mono-blades are available for installation on the lower section.
Fig.7: Latest design of pusher withstanding more than 1500 heats
Since 2016, compact Condoor® design has been available (overall height only 1815
mm) which can fit most of the EAF bigger than 50 Tons and shaft furnace with fixed
roof. Installation of Condoor® does not require a long shutdown and can be clubbed
with other routine stoppages of the plant.
Process Analysis with Condoor®
Optimization of slag Residence Time is the key to maximum utilization of fluxes and
ferrous raw materials. By enhancing the slag volume inside the EAF, Condoor®
ensures better mixing of slag builders resulting in a better efficiency of slag.
Fig.8: Slag Volume can be increased by about 100% with Condoor®
Equally important is the knowledge of the slag quantity inside the EAF. Condoor® can
help the operator choose:
1. The exact time of deslagging for optimization of Residence Time
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2. The flow rate of deslagging by creating the same size/shape of door opening
every time
Figure 9 shows a detailed melting profile for 50% Scrap and 50% HBI charge with one
deslagging window at the end. With a conventional slag door, deslagging starts in the
middle of the process and continues till the end. Condoor® squeezes this window by
more than 10 min, enhancing the slag Residence Time by the same measure. In such
EAFs the total deslagging time is often less than one minute because of a wide and
clear door opening.
Fig.9: Melting Profile with single deslagging
An EAF with Condoor® opens the door only few minutes before tapping to make the
best use of the slag and minimize furnace cooling by air ingress through the door.
Condoor® brings savings in
Power Consumption
Electrode consumption
Raw materials (higher Liquid Metal yield)
Process time
as demonstrated by 9 installations worldwide (limited to Version 2.0 only).
In case of HM/DRI based process, multiple deslagging can be planned to optimize the
Residence Time of slag (and thus slag builders). Condoor® operation can be
programmed into the automatic cycle of the process to achieve consistent results.
While DRI, scrap and flux feed rates are controlled by the EAF Control System, the
Operator must get a feedback on slag foaming while the slag door is closed. SCAD is
the tool designed by the SMS Group, SCAD meaning Slag Control and Detection
system. By analyzing the data from the electrical system, it can provide the status of
slag foaminess in real time based on CAI, Covering Arc Index.
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Fig.10: Foamy slag control with Condoor® and SCAD
Based on the SCAD feedback, EAF
Control System regulates carbon and
oxygen injection to achieve consistent
foaming of slag. Carbon consumption in
EAF is reduced by avoiding an
overdose of injected carbon. This is
particularly important for EAFs with long
flat bath operations.
EAF Control System can use SCAD inputs to achieve maximum DRI Feed rate in
EAFs as the volume and quantity of slag changes gradually. The higher volume of
slag inside the EAF not only improves the heat transfer efficiency, it also reduces the
requirement of slag builders. As the weight of slag in the slag pots is reduced, so is
the loss of iron and the LM yield improves.
In EAFs using maximum Hot Metal, a different strategy can be adapted to get rid of
the first slag after the entire silicon is oxidized. The door can be closed afterwards as
the oxygen blowing progresses.
Figure 12 shows the changes in slag volume (slag level) during a heat cycle with 100%
DRI charge with feeding rate varying between 1200 to 3500 Kgs/min. With a
conventional door, as the % Charge increases, volume of slag generated increases
with decreasing slag holding capacity in EAF. The window of deslagging is more than
half length of the process, 20 to 25 min for a 52 min tap-to-tap cycle.
Fig.11: Saving in Carbon Injection
-20%
-15%
-10%
-5%
0%
USA SouthAmerica
Asia
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Fig.12: Changes in slag holding capacity with Condoor®
With Condoor®, more than one deslagging step can be planned for the optimal
utilization of slag. The first deslagging can be positioned in a way to make full utilization
of the lime-rich slag from the previous heat. The Operator can keep track of the inside
slag quantity from the material input data and the exact time of deslagging. The second
deslagging step can be merged with the final temperature and sample collection. With
the tunnel sealed completely by Condoor®, there is no need to tilt EAF backwards,
thus altering the distance between the wall-mounted lance and steel bath. Another
remarkable gain with Condoor® is that the short circuiting between feeding zone and
slag door can be ruled out completely. The Operator stops feeding of lime and DRI
when the door is open. Longer Residence Time of Lime and DRI results in A. smaller
slag generation and B. reduced loss of iron through the slag door.
Experience learnt from a 100% Scrap based 98 Tons AC furnace has shown the
possibility to use the entire volume for slag holding until the so-called Sealed level, in
order to reduce the overall consumption of Electrical energy (-2.9%) and Electrodes (-
6.7%), at the same time increasing the Liquid Metal yield by 0.7%. The winning
strategy has been achieved at the minimum cost of +1% Carbon charged and +1%
slag builders charged.
USA Europe South America Asia
Electrode consumption -6.7 % -15 % N.A. -2.5 %
Electric consumption -2.9 % -1.5 % -1.2 % N.A.
Table 1: Savings in Electric Power and Electrode thanks to Condoor®
Due to the uncontinuous production, most of the plants worldwide find it difficult to
keep consistency on their operation, which is reflecting on high variation of the total
operation time (TTT, tap-to-tap time). This is a new area to look at. As a consequence
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of the market demand, the production is nowadays planned on short notice.
Furthermore, electricity price variations during the day/night force the steel makers to
operate electric arc furnaces only on few shifts per day.
The SMS Group was aware of the importance to provide equipment flexibility in
respect of the raw material charging; today, SMS is also aware that flexibility is
necessary in respect of the production scheduling. Having a short period of time (like
a night-shift) to produce a certain amount of steel calls for a repeatable and precise
operation time, thus a constant tap-to-tap time. A delay of 10 min (not abnormal in a
steel making plant) is not acceptable any longer, just because the average TTT is
mitigated only by 10-12 heats and not by 60-80 heats like in the past. Condoor® is
targeted to grant the highest level of repeatability and operation consistency: a new
approach matching the requirements of modern steel makers.
This is coming mainly from the higher predictability in the power off time: the usual
time losses during sampling and breast maintenance are nearly eliminated. It is quite
impressive to record the increase of consistency of TTT of two EAF plants, namely
100 Ton and 120 Ton heat size in South America and Asia. The measure of this can
be represented by the standard deviation associated with the power off time, a factor
often not analyzed by the steelmakers.
Condoor® stabilizes and decreases the power off time: a remarkable 6 min less per
heat averaged on 200 heats, and above all the elimination of the unpredictable peaks,
which means process consistency at all levels (standard deviation decreased by 20%).
For the South American plant, the stabilization was even higher, with a reduction of
the power off time by 3 min, and a standard deviation decrease by more than 50%
(from 6.2 to 2.7 min).
Fig.13a: 200 heats with conventional slag door on 120 t AC EAF
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Fig.13b: 200 heats with Condoor® on 120 t AC EAF
Data from an EAF plant in USA were analyzed to assess the impact of Condoor® on
the final Phosphorous content of bath. Data of 793 consecutive heats processed
before Condoor® installation was compared with that of 1420 heats made
consecutively after the installation. The result is shown in Figure 14. There was a slight
increase in the average Phosphorous content but the standard deviation was strongly
reduced.[1] Again, process consistency has been improved.
Fig. 14: Change in %P before and after Condoor® installation
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Technological Modules integration
Condoor® is a Technological Module part of the Synergy control platform. Together
with the Advanced Electrode Regulation system (AEREG), Slag Control and Detection
system (SCAD), Conso/SiS burner/injector technologies, robotic TS-Pro Sampler,
FEOS optimization modules, Raw Material control modules and Off-gas Analyzer it
enables the steelmaker to achieve the best performance at the lowest cost on existing
and new installations. The platform provides the integration of hardware equipment
and software control modules: it is built on a modular approach, granting a step-by-
step investment strategy for the steelmaker who can decide which solution to prioritize
on the basis of the current results.
Conclusion
The intense use of alternative iron sources on EAF demands better control over slag
which is not possible with the conventional door. The lack of control can lead to the
loss of slag builders and yield, too. Enhancing the Residence Time is the key for a
better utilization of slag.
Condoor® can provide the complete control over de-slagging: combined with SCAD, it
can help the operator optimize several parameters such as carbon injection, flux
consumption and Liquid Metal yield.
Use of Condoor® in EAF brings saving in terms of:
1. Power consumption,
2. Electrode consumption,
3. Liquid Metal yield,
4. Lime and Carbon consumption,
5. Tap to tap time.
Most of the installations experienced stabilization on the total cycle time due to the
capacity of Condoor® to grant repeatable conditions heat after heat.
No significant impact on dephosphorisation was observed during a study of the EAF
process in a melt shop in USA.
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References
1 †Wally Aggarwal, A. Partyka, Mauro Milocco, “Enhanced Slag Door For Electric Arc Furnace (ESD)”